Lithium ion secondary batteries have been widely used recently in electronic devices such as portable digital devices, as their power sources for driving the electronic devices. Particularly in portable devices such as smart phones that have many functions and need to operate under a heavy load, with respect to the batteries used as their power sources, improvement in battery characteristics such as energy density and load characteristics and reduction in weight are expected. Moreover, improvement in long-term reliability and safety are expected.
Lithium ion secondary batteries typically include a battery case made of metal. Reducing the thickness of the battery case to increase the internal volume and packing a power generation element in the battery case at a high density can improve the energy density. Reducing the thickness of the battery case is particularly effective when the battery size is small, because this can increase the internal volume. As another example, instead of the battery case, a metal-resin laminated sheet is used in some lithium ion batteries. In this example, by using this thin sheet to wrap the battery components such as positive and negative electrodes, separator, and electrolyte, improvement in energy density and reduction in weight can be realized.